Not much is known about the nitrogen (N) uptake capacity and N-form preference of tropical trees. In a<br/>
replicated labelling experiment with 15N-ammonium, 15N-nitrate and dual-labelled glycine applied to saplings of six<br/>
tree species from southern Ecuadorianmontane forests, we tested the hypotheses that (1) the saplings of tropical trees<br/>
are capable of using organicNeven though they are forming arbuscularmycorrhizas, and (2) with increasing altitude,<br/>
tree saplings increasingly prefer ammonium and glycine over nitrate due to reduced nitrification and growing humus<br/>
accumulation. Three- to 5-y-old saplings of two species each from 1000, 2000 and 3000 m asl were grown in pots<br/>
inside the forest at their origin and labelled with non-fertilizing amounts of the three N forms; 15N enrichment was<br/>
detected 5 days after labelling in fine roots, coarse roots, shoots and leaves. The six species differed with respect to<br/>
their N-form preference, but neither the abundance of ammonium and nitrate in the soil nor altitude (1000–3000<br/>
m asl) seemed to influence the preference. Two species (those with highest growth rate) preferred NH4+ over NO3?,<br/>
while the other four species took up NO3? and NH4+ at similar rates when both N forms were equally available. After<br/>
13C-glycine addition, 13C was significantly accumulated in the biomass of three species (all species with exclusively<br/>
AM symbionts) but a convincing proof of the uptake of intact glycine molecules by these tropical montane forest trees<br/>
was not obtained.